1. Field of the Invention
The present invention relates to a wireless LAN system for transmitting a signal for occupying a radio medium.
2. Related Background of the Invention
Conventionally, a wireless LAN system in compliance with the IEEE 802.11a (hereinafter, referred to simply as “11a”) standard having a maximum transmission speed of 54 Mbit/s is proposed, in addition to a wireless LAN system in compliance with the IEEE 802.11b (hereinafter, referred to simply as “11b”) standard having a transmission speed of 11 Mbit/s. Moreover, the IEEE 802.11g (hereinafter, referred to simply as “11g”) standard having both features of the IEEE 802.11b and IEEE 802.11a is proposed. Hereinafter, an access point and a station in a wireless LAN system in compliance with these standards are referred to as a “legacy terminal”, a legacy access point as a “legacy AP”, and a legacy station as a “legacy STA.”
In recent years, a transmission speed as high as 100 Mbit/s is required in a wireless LAN system. A method is proposed as one of methods for fulfilling the need, in which the transmission speed is increased by using a frequency band n times (n is a natural number) that of a legacy terminal. Hereinafter, an access point and a station in a wireless LAN system in which such a high-speed transmission is performed are referred to as an “HT (High Throughput) terminal”, an HT access point as an “HTAP”, and an HT station as an “HTSTA”. In FIG. 40, the HT terminal includes a frequency band f1 used by the legacy terminal and uses, for example, a frequency band fh having a band width three times that of the legacy terminal. At present, communication by an HT terminal and a legacy terminal using the same frequency band, that is, the coexistence of different wireless LAN systems is under examination.
Conventionally, examples of coexistence of different wireless LAN systems using the same frequency band include the coexistence of the 11b system and the 11g system. This wireless LAN system employs such a configuration as shown in FIG. 41. In the configuration, an access point 100 of 11g controls both of a station 101 of 11g and a station 102 of 11b.
In the case of this configuration, it becomes possible for the station 101 of 11g to perform a high-speed transmission by transmitting a data frame in the OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme, but the station 102 of 11b recognizes the OFDM signals as interfering signals from other systems because the OFDM signals are not the signals of 11b. Then, depending on a signal level, there may be a case where the station 102 of 11b transmits a data frame in the CCK (Complementary Code Keying) modulation scheme. In such a case, a collision of packets occurs and the throughput is deteriorated.
In order to avoid this, as shown in FIG. 43, before transmitting an OFDM signal, the station 101 of 11g transmits a CTS (Clear To Send: a signal to notify the completion of reception) in the DSSS (Direct Sequence Spread Spectrum) modulation scheme, which can be demodulated by the station 102 of 11b and a station, which is not shown, in compliance with the IEEE802.11 (hereinafter, referred to simply as “11”). Due to this, an NAV (Network Allocation Vector: suppression of transmission by a virtual carrier sense) is set to the station 102 of 11b and the station of 11, which is not shown, thereby a collision is avoided.
Here, the frame format of CTS employs a configuration shown in FIG. 44. That is, the frame format consists of a frame control section 103, a duration section 104, and a receiver address 105 as an MAC (Media Access Control) header, and an FCS (Frame Check Sequence) section 106. Other terminals that have received such a CTS refrain from performing transmission by setting an NAV in the interval described in the duration section 104. Due to this, a collision is avoided.
Also in the case where the station 102 of 11b has already performed transmission, it is possible for the station 101 of 11g to demodulate or recognize the signal transmitted from the station 102 of 11b, therefore, a collision can be avoided. In other words, as shown in FIG. 45, the station 101 of 11g demodulates (or recognizes) the data frame (CCK) transmitted from the station 102 of 11b and sets a backoff to avoid a collision, then transmits an OFDM signal.
[Non-patent document 1] “System Capacity Comparison with various High Data Rate Wireless LAN Systems”, Satoru Hori et al., The 2002 IEICE (Institute of Electronics, Information and Communication Engineers) General Conference, B-5-250
[Non-patent document 2] “802.11 High-speed Wireless LAN Textbook”, Hideaki Matsue et al., 2003, IDG Japan
[Non-patent document 3] IEEE Std 802.11g-2003